During the past 20 years, permanent magnet alloys of high field strength and high resistance to demagnetisation have been developed, these alloys permitting the use of magnets in applications not previously considered possible. A typical example resides in the use of permanent magnets to hold dentures in place in the mouth, the magnets being located in the dentures and being attractively coupled to keepers that are attached to teeth roots or implants. Magnet alloys in common use for this purpose are those based on cobalt and samarium, and iron, neodymium and boron.
A number of different arrangements employing closed field permanent magnets and pole pieces have been proposed for use in denture retention systems. In this regard reference may be made to U.S. Pat. Nos. 4209905 and 4302189 (both to Gillings), 4431419 (Portnoy), 4508507 (Jackson) and 4626213 (Shiner). These closed field arrangements have been adopted in favour of more elementary (open field) retention systems because they confer two advantages. In the first place, they reduce substantially the external magnetic fields which are associated with open field magnet systems. Secondly, the reduction in external magnetic fields is usually accompanied by a considerable increase in the retention provided.
The reduction in external magnetic fields is desirable because some authorities believe that there is a remote possibility that such fields may have noxious effects on living tissues. The increase in available retention is desirable because it enables a denture to be held in place more firmly or, alternatively, allows the construction of devices that are much smaller than those that employ open field magnet elements.
However, a disadvantage of the presently available high strength magnet alloys is that they are susceptible to corrosion when used in the mouth if not protected against oral fluids. The corrosion products are not toxic or damaging to denture wearers, but the effect of the corrosion is to reduce available retention and, in some cases, this reduction may be so great that retention is lost completely. For this reason, most commercial manufacturers seek to protect the magnetic denture retention devices against corrosion. Plastic and electroplated coatings have been tried but have been found to be unsatisfactory and some (if not all) manufacturers currently enclose their magnets in thin corrosion-resistant metal casings.
However, the devices which have encased magnet elements can be difficult to manufacture and/or have other disadvantages. One such device employs a non-magnetic sleeve to encase the periphery of its magnet element, and non-corrosive magnetisable material side plates are glued to the opposite pole faces of the magnet element, such that the entire surface of the magnet element is covered by either the sleeve or the side plates. The side plates themselves constitute north and south pole pieces which in use abut a keeper to form a closed field magnetic system. Although this device has been used extensively and sucessfully, it is known that excessive biting forces can cause the glued joint to fail, allowing ingress of oral fluid and resultant corrosion.
Another magnet system which currently is marketed has a cylindrical magnet which is capped at both ends by concentrically disposed inverted cups. The lip of one of the cups extends to and surrounds the base of the other, such that concentric pole faces are created, one being circular and the other being annular. However, this structure does have a poor retentivity-to-size ratio.
A further known magnet system employs a cylindrical magnet element which is positioned within two recessed hemispherical keeper plates, with the plates being separated by a small gap. The plates form north and south pole units and the gap between the plates is filled with a non-magnetisable low melting temperature solder. This particular product does exhibit high retentivity but it is bulky and is expensive to produce. Also, the solder can wear or corrode in the oral environment.